Newton's cradle is a ball collision demonstration device that demonstrates conservation of momentum and energy via a series of swinging balls. Newton's cradle is a popular and well known amusement device and may be found sitting on office desks, for example, around the world.
A typical Newton's cradle includes a series of identically sized metal balls suspended from a frame so that they are just touching each other at rest. Each ball is attached to the frame by two lines of equal length angled away from each other. This restricts the pendulums' movements to the same plane.
If a first ball is pulled away and released, it strikes the next ball in the series and comes to nearly a dead stop. The last ball on the opposite side of the series acquires most of the velocity and almost instantly swings in an arc almost as high as the release height of the first ball. This shows that the last ball receives most of the energy and momentum that was in the first ball.
The impact produces a shock wave that propagates through the intermediate balls between the first and last balls. Any efficiently elastic material, such as steel, will do this as long as the kinetic energy is temporarily stored as potential energy in the compression of the material rather than being lost as heat. If two balls, for example, are pulled away and released, then two balls on the opposite side swing out and back.
Over the decades, there have been various design variations to Newton's cradle. For example, U.S. Pat. No. 5,158,462 to Hones et al. discloses a ball collision device comprising several balls hanging from a support structure. Each ball has a different mass, and is arranged in close proximity in order of decreasing mass with their centers lying along a horizontal straight line. When the heaviest ball, hanging at one end of the line, is pulled back and released, the resulting impact leads to a transfer of energy through the line of balls to the lightest one, at the other end of the line, which is accelerated to a high velocity. When appropriately directed, this high velocity can cause the lightest ball to rise to a much greater height than that from which the heaviest ball was released. The device does not permit the lightest ball to rise to its full possible height, and instead, uses a wind-up bar to intercept its rise and cause it to return to its rest position.
Another variation of Newton's cradle is disclosed in U.S. Pat. No. 3,594,925 to Abbat. Abbat discloses an amusement device comprising a generally parallelepiped-shaped frame member in which a plurality of spherical mass retaining yokes are freely suspended. The frame member includes a pair of side plates detachably connected by a plurality of elongated braces. The mass-retaining members are detachably mounted in the frame member, whereby the entire device may be assembled in a relatively compact package for carrying or storing.
Yet another variation of Newton's cradle is where the support structure includes a pair of spaced apart rails, and each rail is curved upwards. As a result, the lines attached to the balls have longer lengths at the opposing ends of the rails as compared to the lines at the middle of the rails.
Even in view of the above variations on Newton's cradle, there is still a demand for further variations, as evidenced by the popularity of such ball collision devices.